The invention relates to a method and an apparatus for internally coating a metal tube, particularly a weapon barrel, with an electrically conducting layer.
In weaponry, efficiency-augmented ammunition causes, because of the high gas temperatures and flow velocities generated during firing, particularly in steel weapon barrels, significant erosions which wear out the barrel even before service-terminating metal fatigue sets in. It has been known to provide the inner face of weapon barrels with a hard chromium layer to prevent such erosions. Conventionally, the hard chromium is electrolytically deposited on the inner surface of the weapon barrel.
It is a disadvantage of the above-outlined conventional method that, among others, the electrolytically deposited hard chromium layers are very brittle and are not capable of withstanding the stresses caused by efficiency-augmented ammunition. Thus, significant erosion phenomena appear at barrel areas which consequently lose the chromium coating.
It is an object of the invention to provide an improved method and apparatus of the above-outlined type with which particularly metals of high melting point may be applied in a simple manner to the inner face of a metal tube and wherein the coating applied to the metal tube has a high degree of bonding strength.
These objects and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the method of coating an inner face of a metal tube with an electrically conducting coating material includes the steps of positioning a wire, made of the coating material, generally axially in an interior of the tube, along the inner face thereof; and passing an electric current pulse of sufficient intensity through the wire to cause an explosive vaporization thereof, whereby particles of the vaporized material impinge on the inner face of the metal tube and form a layer thereon.
It is thus the essence of the invention to provide an inner coating on a tube by a xe2x80x9ccoating from the vapor phasexe2x80x9d in which the vaporization of the coating material is effected by a xe2x80x9cwire explosionxe2x80x9d.
For performing the method, first the coating substance is, as a wire, arranged generally axially in the tube or tube section to be coated. Thereafter, an electric current pulse is passed through the wire, wile selecting the current intensity such that the wire vaporizes in a burst. The particles obtained by vaporizing the substance impinge with a high kinetic energy on the inner surface of the tube causing a local melting of the particles on the tube wall and a formation of an alloy between the tube material to be coated and the coating substance. In this manner, a high degree of bonding strength between tube and coating is obtained.
It is an advantage of the method according to the invention that it may be used for a great number of diverse coating substances. Further, the coating applied with the method according to the invention has a lesser porosity as compared to methods applied by means of thermal spraying.
To ensure that the particle speed of the particles upon vaporization of the coating substance is not braked by air molecules, it is advantageous to perform the coating operation in a vacuumized inner space of the tube. Such a method has further the advantage that both cluster formations and oxidation on the inner tube surface are avoided.
According to another embodiment of the invention, it has been found expedient not to evacuate the inner space of the tube but to introduce a working gas thereinto. In this manner, the hardness of the coating is further increased. It has been found to be particularly advantageous to use methane as a working gas in conjunction with carbide-forming coating substances.